1. Field of the Invention
The present invention relates to inductive output amplifiers having RF modulation applied to an electron beam passing through a grid disposed between an electron emitting cathode and an anode. More particularly, the invention relates to a low impedance structure that prevents self-oscillation of the electron beam at a frequency determined in part by the resonant frequency of the grid-anode interaction region.
2. Description of Related Art
It is well known in the art to utilize a linear beam device, such as a klystron or travelling wave tube amplifier, to generate or amplify a high frequency RF signal. Such devices generally include an electron emitting cathode and an anode spaced therefrom. The anode includes a central aperture, and by applying a high voltage potential between the cathode and anode, electrons may be drawn from the cathode surface and directed into a high power beam that passes through the anode aperture.
One class of linear beam device, referred to as an inductive output amplifier, or inductive output tube (IOT), further includes a grid disposed in the inter-electrode region defined between the cathode and anode. The electron beam may thus be density modulated by applying an RF signal to the grid relative to the cathode. After the density modulated beam is accelerated by the anode, it propagates across a gap provided downstream within the inductive output amplifier and RF fields are thereby induced into a cavity coupled to the gap. The RF fields may then be extracted from the cavity in the form of a high power, modulated RF signal.
As the modulated electron beam passes through the interaction region defined between the grid and the anode, the modulated beam will radiate RF energy from the interaction region if a high enough impedance is presented to the modulated beam. Ideally, by avoiding reflections of the RF energy and surrounding the grid-anode interaction region with "free space," a low impedance is presented which minimizes RF radiation from the interaction region. In practice, however, there is some leakage of RF radiation from the grid-anode interaction region which can be harmful to other equipment and persons in proximity to the device, and can couple to the cathode-grid space causing oscillation. To prevent such undesirable leakage, the device is ordinarily enclosed within a metallic housing which effectively shields the RF radiation.
An unintended consequence of the housing, however, is that it necessarily forms a cavity connected to the grid-anode interaction region. If this grid-anode cavity presents a high impedance to the modulated electron beam, the beam will radiate RF energy into the grid-anode cavity which may be coupled back into the cathode-grid space. This can cause undesirable regeneration of the beam modulation, i.e., a self-oscillation condition in which the electron beam is further modulated at a frequency determined by the resonant frequencies of the cavities. The unwanted modulation of the electron beam interferes with the RF signal which is desired to be amplified by the inductive output amplifier, and the radiated RF energy reduces the power of the modulated beam, which reduces the gain of the amplifier. In extreme cases, the self-oscillation can generate voltages high enough to damage the amplifier.
An approach to overcoming this self-oscillation problem is to load the cavity with lossy material in order to present a low impedance to the electron beam over the band of frequencies at which the inductive output amplifier operates. As known in the art, ferrite loaded silicone rubber material presents a low impedance in the UHF and microwave frequency ranges and is capable of standing off very high DC voltages on the order of several tens of kilowatts. A drawback of the use of such lossy material is that it is labor intensive, and hence costly, to apply the material to the grid-anode interaction region. Moreover, the high voltage standoff characteristics of the material tend to degrade over time, which reduces the performance of the inductive output amplifier.
Thus, it would be desirable to provide an inductive output amplifier having a low impedance grid-anode interaction region which avoids self-oscillation. It would further be desirable to avoid the reliance upon lossy ferrite material in reducing the impedance of the interaction region.